The field of protective coatings constitutes an important use for polymeric materials, especially organic polymers. Almost from the inception of polymer science it was recognized that a polymeric coating could protect an otherwise sensitive underlying substrate from an "unfriendly" environment. Of course, this was recognized years before in various "natural" settings; e.g., aluminum is oxidation resistant principally because a thin film of alumina readily forms on all surfaces exposed to air and thereafter acts to inhibit bulk oxidation.
The rapid development of integrated circuit (IC) technology from small scale integration to very large-scale integration (VLSI) has had great technological and economic impact in the United States. The exponential growth of the number of components per IC chip and the similar decrease of minimum device dimensions have imposed stringent requirements, not only on the IC physical design and fabrication but also on the IC encapsulants.
The purpose of encapsulation is to protect electronic IC devices from moisture, ionic contaminants such as mobile sodium, potassium, and chloride ions, uv and alpha particle radiation, and other hostile operating environments including corrosive agents such as acids, bases, and oxidizing agents. In addition, encapsulation enhances the mechanical and physical properties of fragile IC devices and improves manufacturing yields and reliability. The encapsulant must be an ultrapure material with an excellent barrier to moisture and contaminant mobile ions with superior electrical, physical, and mechanical properties, and which is easy to apply and to repair in production. See Encyclopedia of Polymer Science and Engineering, V 5, pp 638-641, J. Wiley & Sons (1989). A variety of encapsulants, both inorganic and organic polymers, are available having a broad spectrum of properties, and generally the choice of encapsulants is made depending upon necessary properties which are a consequence of the working environment. Thus, a need for outstanding solvent resistance may dictate one type of encapsulant, whereas a need for high temperature stability may require a quite different type of encapsulant. As a general proposition it can be fairly said that the nature and properties of available encapsulants are sufficiently well known that the worker in this field can make an intelligent choice which will be well suited to the task at hand. However, occasionally particularly harsh environments make extreme physical and/or chemical demands which remain unmet by conventional materials. This application is directed to one sort of such extreme; our invention is a solution to the unusual demands of a particularly unfriendly environment.
Our specific needs arose from the problem of acids permeating through an elastomeric encapsulant to attack aluminum deposited as a metallization layer on a silicon chip, ultimately resulting in device failure through bond pad corrosion. We will describe our invention, and the problem which it solves, in the specific context in which it was encountered. However, we emphasize at the outset that our solution is a general one; it can be applied to a wide variety of organic polymeric protective coatings to impart additional protection in acidic, basic, and oxidative environments. It also should be apparent that our invention is directed to the organic polymeric material, thus is independent of the device encapsulated by the polymer. Nonetheless, solely for clarity and simplicity of exposition we shall describe our invention in the context of a particular device.
The problem whose solution is the invention of this application arose in connection with a piezoresistive transducer used as the pressure sensor on the intake manifold of an internal combustion engine. The most critical use of information from such a pressure sensor is to regulate the amount of fuel supplied to the engine by the fuel injectors, and the sensor is an integral part of the total emission control and fuel economy in automotive vehicles. The manifold absolute pressure sensor is widely held to be second in importance only to the ignition pickup device in proper operation of internal combustion engines.
The piezoresistive transducer in question, independent of associated electronics, is a monolithic silicon chip. The output voltage varies with pressure via a resistive element, acting as a strain gauge, implanted on a thin silicon diaphragm. The resulting die is aluminum metallized, with bond pads for, e.g., gold wire bonds to connect the chip to the lead frame and the die is mounted on a room temperature vulcanizing (RTV) elastomer, exemplified by a silicone or fluorosilicone, to the package base. The die, including the wire bond pads, is encapsulated in a protective polymer coating. Since the function of the die is to measure pressure, the encapsulant must not be a rigid, non-deformable polymer but instead must transmit external pressure changes to the die. Hence the encapsulant is an elastomer, which can be described variously as a rubbery, gummy, or gel-like material, and silicones, including fluorosilicones, are the elastomers of choice, in part because of their excellent resistance to water and hydrocarbon vapors which are a normal incident of the environment in which the manifold absolute pressure sensor is placed. Several configurations are possible for encapsulant(s) within the housing; these variations are unimportant for the present discussion although they will be referred to in greater detail within (vide infra).
The problem which arose was an unexpected failure of the piezoresistive transducer under certain conditions. Upon further investigation it was determined that failure occurred only in the highly acidic environments associated with high concentrations of, for example, HNO.sub.3 and/or H.sub.2 SO.sub.4. Closer examination showed that the failure arose specifically via corrosion at the wire bonds. Consequently, the problem was that acids or acidic gases of nitrogen and/or sulfur were permeating through the silicone elastomer used as the encapsulant and attacking the aluminum deposited during metallization, specifically at the bond pads. The aluminum layer elsewhere had a silicon nitride passivation overcoat which acted as a protectant, so only the bond pads were susceptible to corrosive attack.
The problem thus is clearly defined and its origins are well understood. Since no control over the environment is possible and since it is not feasible to further protect the aluminum at the bond pads, the solution to the problem must be directed either to eliminating or reducing permeation of acids through the encapsulant, or to neutralize acids within the encapsulating polymer. Our solution to the problem is to disperse a solid buffer within the elastomer, which certainly serves to neutralize acids but, depending upon how the elastomer is used, also may prevent permeation of the encapsulating elastomer by offending acids.
Although the foregoing background to our invention is couched as a specific problem associated with a specific device, the general problem and its solution may be readily appreciated. Thus, there is a wide spectrum of environments where the concentration of acidic or basic materials is sufficient that, upon their permeation through a protective polymer layer, the protective encapsulant and/or the underlying device may be attacked leading to corrosive failure. The general solution is to disperse in the protective polymer a solid acting as a buffer with respect to the corrosive agent.
Throughout this application we use "buffer" in its most extended chemical sense, i.e., a substance which resists a change in hydrogen ion concentration upon addition of acids or alkali, or a substance which resists a change in oxidation potential. The former is more generally intended within this application; this class is referred to as "acid-base buffers." Unless "oxidation-reduction buffer" is specified "buffer" shall mean an "acid-base buffer."